In the field of printing machines, the application of a printing image in correct positional arrangement on a printing substrate is of considerable importance regarding the printing quality. Any misaligned printing of one or more colors on the printing substrate is readily perceived by the human eye and felt to be distracting.
Therefore, it is generally known that printing machines are regularly calibrated. In particular, it may be necessary to perform a calibration for each printing substrate that is being used. This means, when different printing substrates are used, a calibration must be performed for each individual printing substrate.
For such a calibration, as a rule, a plurality of registration marks are printed on a transport belt of the printing machine and on the respective sheets of the printing substrate during a calibration process. If the printing machine comprises a duplex device, registration marks are printed on the front side (recto printing) as well as on the reverse side (verso printing) of the respective sheets and are subsequently measured by means of a registration sensor.
If the printing substrate is partially transparent, the calibration cannot be performed in its usual way because the registration marks printed on the front side may show through, thus impairing a measuring of the registration marks on the reverse side of the sheet by means of the registration sensor.
To solve this problem, application DE 10 2004 007 367 that goes back to the applicant has suggested a method wherein, while the front and reverse sides are being printed, the sheet is offset transverse to its transport direction through the printing machine in order to print the respective registration marks offset with respect to each other on the front and reverse sides of a sheet. This can prevent the measurement of the marks by means of the registration sensor from being disrupted; however, in part, this method is highly complex in view of the required offset of the sheet, and an appropriate device allowing sufficient transverse offset of the sheets must be provided.